catch trouble that materializes only when we destroy the VGZ instance.

Fixes	#1037
Fixes	#1043
Fixes	#1044

Conflicts:

	bin/varnishd/cache.h
	bin/varnishd/cache_center.c
	bin/varnishd/cache_esi_deliver.c
	bin/varnishd/cache_esi_fetch.c
	bin/varnishd/cache_fetch.c
	bin/varnishd/cache_gzip.c
	bin/varnishd/cache_response.c